DC motors are widely used in many industrial and consumer applications. In many cases absolute precision in movement is not an issue, but precise speed control often is. An example of this would be the drive motor of a cassette player. The cassette player is expected to run at a constant speed and so speed control is important, but the motor does not have to run for precise increments which are fractions of a turn, or to stop at a certain point.
However, some motor applications do require precise positioning. For example, the motors used in high resolution plotters, disk drives, and robotics must all be carefully and precisely controlled in terms of position. There are also a number of applications that require both precise speed control and some position accuracy. Video recorders, compact disc drives and high quality cassette recorders are example of this type of device. Furthermore, by controlling DC motors accurately they can overlap with many of the applications of stepper motors.
The cost of the control system depends on the accuracy of the feedback and the speed of the processor. There are two general types of motor control systems: open loop systems and closed loop systems. In an open loop system the controller outputs a signal to turn the motor on and off or to change the direction of rotation of the motor. Closed loop systems are similar to open loop systems but in addition involve a feedback signal to the controller from the motor which feedback signal carries information about the speed, and/or position and/or the direction of motion of the motor. An example of a closed loop system is one where the motor must run at a constant speed (eg a cassette recorder) where the controller constantly adjusts the speed of the motor to keep it within limits and uses a feedback signal from the motor to do this.
The feedback signal may come from a sensing device adapted to sense motion. Examples of possible sensing devices include optical encoders, infrared detectors, Hall effect sensors and many others. Depending on the application, one or more of such sensors may be chosen. However, they all have their own disadvantages. Optical sensors, for example, often have to be provided with some form of housing or encapsulation to prevent a loss of sensitivity from ambient light, dust and dirt and so on. For Hall effect sensors, in practice the gap between a magnet that is mounted on the motor rotor and the sensing device is often too large for accurate and reliable results.
A traditional electric motor comprises an armature bearing three windings. The armature rotates in a magnetic field and current is generated in the three windings and drawn from them in turn through brushes.
Such a conventional motor is shown in FIG. 1A. A motor armature comprises a rotor having three equiangularly spaced poles 1 about each of which is wound a coil winding .PHI.. Coil windings .PHI. are connected to commutator segments 2 which in turn are contacted by brushes (not shown). Such a motor may be used to cause rotation by applying current to the windings which then rotate within a magnetic field, or may be used in reverse to generate current from rotation of the windings within the magnetic field.
In a conventional motor the three coil windings .PHI.1, .PHI.2 and .PHI.3 are all identical and have identical numbers of turns in each winding. When the motor rotates, the current that flows through each winding is therefore identical. One way of providing a feedback control signal is to form the three coil windings .PHI. with a differing numbers of turns. This is shown in FIG. 1B where winding .PHI.1-1 is formed with a reduced number of turns in comparison with windings .PHI.2 and .PHI.3. The effect of this is that as the motor rotates the current that flows through winding .PHI.1-1 is different from that which flows through windings .PHI.2 and .PHI.3. This difference can be detected and used to count the number of rotations of the motor, and also to mark and define the beginning of rotation cycles of the motor. This information can be used in a number of ways to accurately control the rotation of the motor in a number of applications as discussed above. Furthermore this feedback control system has the advantage of not requiring an additional sensor, instead the information of the actual motor speed and the position of the motor can be extracted from the motor itself. In effect, a DC motor is provided with an odd number of poles which generate a motor current containing a fixed number of discontinuities which define a signature wave form. This allows speed information and position information to be derived from the motor current and thus allows it to be used, for example, as a replacement for a low resolution AC tachometers system
A disadvantage of forming one winding with fewer turns than the other windings is that a mechanical imbalance is introduced. As can be seen in FIG. 1A, when the three windings .PHI. are all identical, the mechanical centre of gravity 3 of the rotor coincides with the axis of rotation of the rotor. In FIG. 1B, however, the centre of gravity 3 is moved off the axis of rotation in a direction away from the reduced turn winding .PHI.1-1. This mechanical imbalance inevitable introduces a number of difficulties and problems, including noise and excessive wear on the rotor bearings.